Process for controlling the manufacture of high-chromium steels

ABSTRACT

THE REFINING BY OXYGEN BLOWING OF HIGH-CHROMIUM LOWCARBON MOLTEN STEEL IS CONTROLLED BY MEASURING THE TEMPERATURE OF THE METAL DURING BLOWING, WHICH TEMPERATURE IS RELATED TO THE CARBON CONTENT.

April 20, mm

BATH TEMPERATURE T. PF)

F. REHMus ET AL 3,575,696

PROCESS FOR CONTROLLING THE MANUFACTURE OF HIGH-CHROMIUM STEELS Filed Sept. 19, 1968 .04 .os .08 J0 .12 .14 J6 BATH CARBON /0,

INVENTORS FREDERICK H. REHMUS JULIUS D. SHIMKETS BY EGIL AUKRUST DAVID.H WA KELIN their ATTORNEY United States Patent 3,575,696 PROCESS FOR CONTROLLING THE MANUFAC- TURE 0F HIGH-CHROMIUM STEELS Frederick H. Rehmus, Baldwin Borough, Julius D. Shimkets, Pittsburgh, Egil Aukrust, Bethel Park, and David H. Wakelin, Pittsburgh, Pa., assignors to Jones &

Laughliu Steel Corporation, Pittsburgh, Pa.

Filed Sept. 19, 1968, Ser. No. 760,774 Int. Cl. C21c /32 US. Cl. 75-60 4 Claims ABSTRACT THE DISCLOSURE The refining by oxygen blowing of high-chromium lowcarbon molten steel is controlled by measuring the temperature of the metal during blowing, which temperature is related to the carbon content.

The high-chromium steels with which this invention is concerned are those generally known as stainless steel. The great bulk of these are encompassed in the A181 300 and 400 series, so-called, the steels of the firstmentioned series containing nickel in amounts of about 8% or more, and the steels of the latter no nickel or nickel only in amounts less than about 2 /2%. Both the 300 and 400 series steels contain chromium in amounts from 16% to as high as 27%.

High-chromium steel are conventionally made by melting a scrap charge high in chromium in an electric arc furnace. In recent years it has become convent1onal to oxidize carbon from the molten metal by blowing it with substantially pure oxygen. The scrap charge is inherently low in carbon so that the carbon content of the bath on meltdown is seldom over about .5% and may be less. Many such high-chromium steels have carbon specifications of about .08% maximum, and few have maximum carbon specifications over .15%. It is necessary to reduce the carbon of the steel in the furnace to a level below the specified maximum in order to allow for the carbon content of alloying additions and to provide a margin of tolerance. If oxygen is blown into the bath at a substantial rate, on the order of 40 to 50 cubic feet per minute per ton of charge, the carbon is oxidized to the desired level in a period of time on the order of 15 to 20 minutes.

A very substantial portion of the chromium 1n the bath is oxidized by this oxygen blow and goes into the slag. Procedures have been developed for reducing a part of the Cr O of the slag to metallic form, and these practices, of course, increase the time required for a heat and the cost of making it. It has also been found that oxygen blowing in electric arc furnaces tends to cause quite severe erosion of the furnace roof refractories. For all these reasons, it is desirable to minimlze the length of the oxygen blow.

The carbon content of the bath is conventlonally de termined by taking samples from the bath and analyzing them by known methods. Rapid methods of analysis have been developed in recent years, but the time required for sampling and analysis is still such that, prior to our invention to be described, overblowing of the bath could not consistently be avoided in commercial steelmaking operations.

It is an object of our invention, therefore, to provide a process of manufacture for high-chromium steels in which the oxygen blow is terminated as soon as the bath carbon reaches an acceptable level. It is another object to provide a process of rapidly determining the carbon content of the bath when that carbon content is at a low level. It is still another object to provide a process of determining the carbon content of the bath in advance of the desired end point and of predicting therefrom the quantity of oxygen required to bring the carbon content of the bath to its desired level. Other objects of our invention will appear in the course of the description thereof which follows.

Conventional electric furnace practice for the manufacture of stainless steels and the like will be illustrated by reference to the practice for making 300 series steels in a steelmaking shop with which we are familiar. This shop is provided with 60-ton electric furnaces. The charge for these furnaces is divided into two portions. The first charge, comprising approximately 25 tons, consists of stainless scrap, stainless pig and carbon steel scrap. The composition of the charge is influenced considerably by the relative availability of its constituents. Power is then applied to the furnace and the first charge is melted, which takes a period of time from about 1 /2 hours to about 2 hours. The second scrap charge of 25 tons is then made. This is similar to the first charge but usually includes some nickel oxide and high-carbon ferrochromium. The second charge is melted by the electric arc, again requiring about 1 /2 to about 2 hours. When the bath is fully melted, it usually contains from about 3% to about .8% carbon and about 13% to 15% chromium. 'Ihe chromium content is thus many times greater than the carbon content. The temperature of the bath at this time is usually from about 2800 to 2900 F.

Oxygen is then blown into the charge through a lance to oxidize the carbon to the desired level. Current practice in this shop is to feed oxygen at the rate of about 2500 to about 3000 cubic feet per minute, and the length of the oxygen blow ranges from about 15 to 20 minutes. The oxygen oxidizes the carbon in the bath and also a very substantial amount of the chromium. At the end of the oxygen blow, the furnace slag contains as much as 40% to 50% chromium. The slag is then treated with reducing agents to reduce as much of the Cr O as possible into metallic chromium in the bath. After this treatment, the slag is poured off into slag pots. Lowcarbon ferrochromium and ferromanganese additions are then made, the bath is sampled, and any other alloying agents required to bring its composition to the desired analysis are added.

We have found that after 9 or 10 minutes of oxygen blowing at the rate above mentioned, the carbon content of the bath is almost always below 0.10%. We have found that the increase in bath temperature occasioned by the oxygen blowing accelerates markedly as the bath carbon content falls below 0.10%, and at bath carbon contents of 04% to .05% reaches 3400 to 3500 F. Such high temperatures can well account for the roof refractories damage which has been experienced with oxygen blowing in electric furnaces.

We have further found that below bath carbon levels of .10% or thereabouts the bath temperature measured during oxygen blowing is uniquely related to bath carbon and that the latter is indicated by the former to a tolerance of L.01%. This bath carbon-temperature relation is shown graphically in the attached figure. The relation there graphed is expressed algebraically as:

3 perature during the oxygen blowing period and stopping the oxygen blowing when the measured bath temperature corresponds with the desired final bath carbon content as determined from Equation 1 above set out.

Another preferred embodiment of our invention comprises the determination of the volume of oxygen required to be blown to lower the carbon content of the bath from a value found by bath temperature measurement as above described to the desired final bath carbon content, and then blowing the bath with that much oxygen. This embodiment requires that the conditions of oxygen blowing be standardized, that is, that the oxygen nozzle diameter and nozzle position be fixed, as well as the oxygen pressure and flow rate. It also requires that the amount of oxygen required for an incremental increase in bath temperature be determined for the particular furnace employed. This value, of course, is influenced by the capacity of the furnace and its heat losses and must be experimentally determined from information obtained from past heats made in the furnace in question.

We have found that carbon is preferentially oxidized until the carbon level falls below about 30%, depending on the blowing practice. From this point on carbon diffusion in the metal becomes the rate determining step in the oxidation of carbon and progressively more chromium is oxidized for each unit of carbon removed. As in a high chromium bath there is never a deficiency of chromium, the heat generated by chromium oxidation per unit of carbon removed increases in a continuous manner. The heat loss characteristics of the furnace will determine the rate of temperature rise of the metal bath. However, for a given furnace and constant blowing practice a unique oxygen consumption-bath temperature relation obtains. Furthermore, after a bath temperature in the range under consideration has been measured, the blowing of a given amount of oxygen will result in a corresponding temperature increase which will not vary within rather close limits. Thus, the oxygen required to go from a higher carbon level to a lower carbon level in the bath, that is to say, from a lower temperature to a higher temperature, can be described as a function of the existing bath temperature.

The amount of oxygen in standard cubic feet required to raise the temperature of the bath from a value measured near the end of the blow to the desired final value is related to those temperatures by the following empirically determined equation:

F Q) where T is the measured temperature of the bath in degrees Fahrenheit, T is the final temperature corresponding to the final carbon content as determined by Equation 1, T is a constant which is, in fact, the temperature corresponding to a bath carbon content lower than the lowest bath carbon content desired in practice, and K is an experimentally determined constant characteristic of the furnace which represents the quantity of oxygen in standard cubic feet required to be blown to increase the temperature of the furnace bath an incremental amount.

We prefer to measure T at a time when about 80% of the oxygen normally required to bring the carbon to its desired final value has been blown. We have found that a desirable value of T is that corresponding to a bath carbon content of about .025 as determined from Equation 1, that is to say, 3620 F. If T is chosen to correspond to some other bath, carbon content A0 will not be greatly changed, as long as T is within the range 3500 to 3750 F. It will be understood that this equation is an approximation and is only applicable under the conditions here set out.

For a 60-ton furnace with which we are familiar, and when T is measured after about 80% of the total oxygen has been blown, the value of K has been determined to be 42,700. As we have indicated this value was found experimentally by measuring the bath temperature at a time when a given amount of the average total oxygen had been blown into the bath, for example, 80%, measuring the bath carbon content at that time, blowing oxygen into the bath at a constant measured rate under constant conditions and measuring the quantity of oxygen blown at subsequent times corresponding to desired values of T,,.

For the measurement of temperatures of the order mentioned herein a rhenium-tungsten thermocouple is suitable. These are available commercially calibrated to temperatures as high as 3800 F. and have a precision of :15. The thermocouple is affixed to the end of a probe or lance and thrust into the furnace bath through the furnace door in the manner well known to open hearth and electric furnace melters.

We claim:

1. In the process of manufacturing chromium-containing steel in a furnace in which a ferrous bath having a carbon content of not more than about .8% and a chromium content many times greater than its carbon content is subsequently blown with oxygen to bring the carbon to a desired final content, the improvement comprising blowing the bath with oxygen to oxidize carbon in preference to chromium, continuing the blowing through the carbon value at which chromium begins to oxidize in preference to carbon, measuring the temperature of the bath during the preferential oxidation of chromium and stopping the blow at a bath temperature which is related to the desired final carbon content by the Equation 1:

been blown, and including blowing the bath with an additional quantity of oxygen which is related to the bath temperature T by the Equation 2:

A02K log (T0 Ta) where A0 is the additional quantity of oxygen in standard cubic feet, T is the bath temperature in degrees Fahrenheit related to the desired final carbon content by Equation 1, T is a temperature in degrees Fahrenheit related by Equation 1 to a bath carbon content lower than the lowest carbon content to which the bath is blown but not lower than about .02%, and K is a constant characteristic of the furnace experimentally determined from previous heats made in the furnace, representing the quantity of oxygen in standard cubic feet required to be blown to increase the bath temperature an incremental amount from the temperature T.

3. The process of claim 2 in which the predetermined proportion of the total oxygen is about 80%.

4. The process of claim 3 in which T is about 3620 F.

References Cited UNITED STATES PATENTS 2,815,276 12/1957 Michaux -60 3,161,499 12/1964 Percy 75-60 3,198,624 8/1965 Bell et al. 7560X 3,336,132 8/1967 McCoy 7560X 3,372,023 3/1968 Krainer et a1 75-60 3,377,158 4/1968 Meyer et al. 756O 3,463,005 8/1969 Hance 7560UX L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner 

